In the Linux kernel, why do many structures use the ____cacheline_aligned_in_smp macro? Does it help increase performance when accessing the structure? If yes then how?
Each cache line in any cache (dcache or icache) is 64 bytes (in x86) architecture. Cache alignment is required to avoid false sharing of cache lines. If the cache lines are shared between global variables (happens more in kernel) If one of the global variables changed by one of the processor in its cache then it marks that cache line as dirty. In remaining CPU cache line it becomes stale entry, which needs to be flushed and re-fetched from memory. This might lead to cache line misses, which requires more CPU cycles. This reduces the performance of the system. Remember this is for global variables. Most of the kernel data strucuters use this to avoid cache line misses.